Automatic disconnection

The Standard IEC 60364 prescribes automatic disconnection of the supply for protection against indirect contact.

What does it mean //

The protective device shall automatically disconnect the supply so that, in the event of a fault between a live part and an exposed-conductive-part or a protective conductor, a prospective touch voltage exceeding 50 V a.c. (25 V in special environments) does not persist for a time sufficient to cause a risk of harmful physiological effect in a person in contact with simultaneously accessible conductive parts.

This protective measure requires co-ordination between the connection to earth of the system and the characteristics of the protective conductors and devices.

This technical article deals with two most common devices suitable for the automatic disconnection of the supply and able to detect earth fault currents, and these are presented below. Note that there are few other types of devices, but they are not mentioned in this article.

Automatic circuit-breakers //

1. With thermomagnetic release

The protections ensured by the automatic circuit-breakers equipped with thermomagnetic release are:

Protection against overloads;

Protection against short-circuits;

Protection against indirect contacts.

Tmax Molded Case Circuit Breaker, type T1

The protection against overload is provided by the thermal release with inverse time-delay curve, i.e. the higher the overload current, the faster the tripping time.

The protection against short-circuit is provided through the magnetic release with an indipendent time trip curve, i.e with disconnecting time independent from the short-circuit current.

The protection against indirect contacts can be carried out both by the thermal release as well as by the magnetic release since the earth fault current involves at least one phase; if this current is high enough, it can cause the tripping of the circuit-breaker.

It is necessary that the protective device is coordinated with the distribution system and the earthing modality of the exposed conductive-parts, so that tripping is guaranteed to occur in such times to limit the persistence of the dangerous touch voltages present in the exposed-conductive-parts further to the fault.

Figure 1 shows an example of the earth fault current path in a system with the neutral is directly earthed and the exposed-conductive-parts are connected to the same earthing arrangement of the neutral (TN system) and the trip curve of a thermal magnetic circuit-breaker type Tmax T1C160 R160.

Figure 1 – Earth current pathFigure 2 – Trip curve Tmax T1C160 In160

As the diagram shows, by assuming an earth fault current of 940 A, the circuit-breaker shall trip in maximum 5s (value read on the curve with the higher tolerance).

Electronic releases allow to get an accurate settings both as regards the trip times as well as the current thresholds so that the installation requirements are fully satisfied. Figure 3 shows the same example as before, but a circuit-breaker type Tmax T2 S160 PR221DS-LS/I In160 with electronic release is installed as protective device.

The possibility of setting a low magnetic threshold (at about 750 A) allows to achieve a trip time corresponding to the magnetic tripping (some tens of milliseconds), which is remarkably quicker than the time obtainable under the same conditions with a thermal magnetic circuit-breaker of the same size.

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Filed Under Category //

Energy and Power Low Voltage

About Author //

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on Google+

there’s an option called LSI-G for the electronic relay on ABB equipments, that “G” parameter is to detect any fault current to ground via a current transformer directly connected to the Power Circuit Braker.